Plaque detecting devices of this sort known in the prior art include, for example, devices which compare the intensity of fluorescent light coming from a tooth surface substantially without deposits (plaque, bacteria, tartar, calculus, etc.) to the intensity of fluorescent light from the tested tooth surface to determine the presence or absence of biological deposits on the tested tooth surface, as disclosed in patent document 1 (Published Japanese Translation of a PCT Application 2002-515276).